The invention relates to a process for mixture preparation in a mixture-compressing externally ignited internal combustion engine in which the fuel metering takes place in dependence on the throttle valve position and the engine rpm. The fuel metering can be performed by carburetors or by fuel injection valves.
Mixture-compressing internal combustion engines must be supplied with the proper amount of fuel corresponding to the aspirated air quantity for each and every power stroke of the engine. The amount of fuel must be such that the combustion produces adequate power but operates without an excess of fuel since that results in an intolerably high degree of toxic components.
For these reasons, it is desired to supply a combustion fuel-air mixture is either at the stoichiometric ratio, where the air number .lambda. equals 1.0 or lies in a region in which there is an excess of air (.lambda.&gt;1.0); the latter condition is particularly suitable to reduce toxic exhaust gas components so as to permit compliance with constantly more rigorous requirements with respect to atmospheric purity. In the following discussion, the mixture preparation system will be understood to be a fuel injection system. In order to correctly adjust the duration of fuel injection, the air quantity aspirated by the engine must be known exactly. This knowledge may be derived from measurement of the air flow rate in the induction tube of the engine, for example by means of a baffle plate which is displaced against a restoring force and serves to adjust appropriate metering means coupled thereto. Unfortunately, this is a relatively expensive process which, furthermore, suffers from the inherent disadvantage that the changes in the filling factor of the cylinder and hence, e.g. the increase of the engine torque, are delayed with respect to the opening of the throttle valve, due to the inertia of the air flow measuring member.
Instead of making an air flow rate measurement, it is also possible to set the fuel injection duration on the basis of the engine rpm and the induction tube pressure. By following the characteristic curve of an induction tube pressure sensor, the correct amount of fuel as a function of induction tube pressure for a particular rpm may be determined.
Induction tube pressure measurements are, however, quite complicated, and, just as in the baffle plate measurement, additional sensors are required. Furthermore, as in the air flow rate measurement, there is a delay in the fuel metering with respect to the changes in air aspiration. A supplementary mechanism is required to achieve a temporary enrichment during a change of the throttle valve position so as to obtain a good transition from one state to the next.
It is relatively simple to obtain a clear signal as to the position of the throttle valve, for example by coupling a suitable potentiometer thereto and, whereas the induction tube pressure changes are delayed with respect to the opening of the throttle valve, the fuel quantity changes at the same time as the throttle valve position. Thus, it is particularly advantageous to determine the fuel injection duration on the basis of the throttle valve position and the rpm. The rpm and the throttle valve positions can also be used to permit an unambiguous indication of the required fuel quantity for each power stroke and this process is also known.
A known characteristic set of curves for a process of this type is shown schematically in FIG. 2 and will be discussed in more detail below. Unfortunately, the fuel injection quantity depends on the rpm and the throttle valve position in a relatively complicated manner. In the function t.sub.i = f(x,n), shown in FIG. 2, t.sub.i is the time during which fuel is injected to a cylinder per power stroke and is therefore proportional to the fuel quantity Q. Since the above mentioned function f is difficult to follow in a direct manner, a known circuit uses a low pass filter in a pulse-shaping circuit to transform this function into a somewhat simpler function which is easier to follow, and this simpler function is subsequently multiplied by another rpm-dependent function. This known method also entails a substantial expense.